Macromolecular Organization of the Cellulolytic Enzyme Complex of Clostridium thermocellum as Revealed by Electron Microscopy

Clostridium thermocellum JW20 and YM4 both synthesize cellulolytic enzyme complexes, cellulosomes, when grown on medium containing cellulose. Electron microscopic studies showed that, in the early stages of growth of strain JW20, clusters of tightly packed cellulosomes, i.e., polycellulosomes, were located on the cell surface and were bound to cellulose. The polycellulosome was estimated to have a particle mass of 50 × 106 to 80 × 106 daltons (Da), while that of the cellulosome was estimated to be 2 × 106 to 2.5 × 106 Da and to contain about 35 polypeptides ranging from 20 to 200 kDa. The cellulosome produced by strain YM4 was found to be somewhat larger, with the estimated particle mass being 3.5 × 106 Da, and the number of polypeptides was counted to be 45 to 50, ranging from 20 to 200 kDa. In the early stages of cultivation, the cellulosomes from both species exist as tightly packed complexes (tight cellulosomes). These subsequently decompose to loosely packed complexes (loose cellulosomes) and ultimately to free polypeptides. Examination of the loose cellulosomal particles showed that they contain rows of equidistantly spaced, similarly sized polypeptide subunits, with an apparently identical orientation arranged parallel to the major axis of the cellulosome. It is postulated that on binding of a cellulose chain alongside such a row of subunits a simultaneous multicutting event occurs that leads to the release of cellooligosaccharides of four cellobiose units in length (C4). Rows of smaller-sized subunits with lower center-to-center distances, which are also present in the cellulosome, subsequently cleave the C4 fragments (or cellulose) to C2 (cellotetraose) or C1 (cellobiose). In this way the cellulosome can catalyze the complete hydrolysis of cellulose.